Boilers comprise tubed walls defining an inner space; a fuel such as coal, oil, waste is supplied into the inner space and is combusted with an oxidizer, such as air or oxygen. During the combustion flue gas is generated that is rich in NOx, whose amount has to be reduced below given limits before the flue gas is vented into the atmosphere.
In order to remove NOx from the flue gas, a regent such as ammonia (NH3 as liquid solution) or urea (CH4N2O as liquid solution) can be injected into the flue gas and the flue gas can be passed through a catalyzer, for the NOx to be converted into Na and H2O (selective catalytic reduction, SCR); this reaction occurs at low temperatures, e.g. between 250-450° C.
The catalyst is expensive and can be damaged during operation, for this reason a method has been developed according to which ammonia or urea are injected into the flue gas having a high temperature, e.g. in the range 750-1300° C., such that the NOx conversion into Na and H2O occurs without the need of a catalyst (selective non-catalytic reduction, SNCR).
WO2012/059 184 discloses a boiler for waste to energy applications (i.e. waste is used as a fuel in the boiler). The boiler has an aperture at its top wall such that a device for selective non-catalytic reduction is introduced into the inner space through this aperture. The device for selective non-catalytic reduction has a tubular lance and a hose sliding into the lance. The hose has a nozzle at its end housed in the inner space. During operation the lance hangs from the top wall of the boiler and the hose extends from the lance; the reagent passes through the hose and is injected through the nozzles upwards.
The reagent after injection has to evaporate to mix with the flue gas, for the mixture to undergo the selective non-catalytic reaction for NOx removal. The inventors have found that evaporation occurs in a limited space around the nozzle; it would be beneficial to have a larger evaporation zone for a larger amount of the flue gas to mix with vapor reagent and thus treat a larger portion of flue gas by selective non-catalytic reaction.